please explain

Ok, I know pickups make a difference but I'm not sure how. Every bass I've ever owned (2) I have always kept the pickups that came with it. I was thinking about buying a new set until I heard how many choices and varieties there was. I also have no idea what sounds they make. I play metal, any suggestions.

IMHO this is where the rubber meets the road. This is one of the most overlooked peices of the signal chain. I think of pickups like tires, If you have a formula 1 racer and you put some cheap ass maypop special tires on it the whole car is a peice of sh*t. To me there are only a few good pup makers out there and everything else is crap. Most people leave the stock pups in and never really experiment with what a good set of pups can do. Most basses that you will find at guitar centers and such will have junk pups and preamps. Im not really a fender fan but I like a few of the p's but to me all fender electronics are junk as with ibanez and so on but this is just "my" opinion. The tonal differencies can be astounding, Like pulling your amp out of the water or something but it does no good to replace cheap factory pups with the $15-$20 specials at your local shop as some of the cheapies are worse than some stock pups. A good pup will bring out the natural tones of your bass. Some pups "color" your sound which can be good or bad depending on what you want. No matter how good the bass if the electronics are junk so is your sound because this is the "link" to your amp.